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Re: [taigtools] Re: milling gypsum ?

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  • Tony Jeffree
    If I interpret the OP s question correctly, he intends to make (female) gypsum molds that he will then use for slip-casting the porcelain; slip-casting
    Message 1 of 8 , Jul 4 6:17 AM
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      If I interpret the OP's question correctly, he intends to make (female)
      gypsum molds that he will then use for slip-casting the porcelain;
      slip-casting involves pouring a slurry (slip) of the porcelain clay into
      the mold, leaving it there while the gypsum draws moisture out of the slip
      & leaves a layer of porcelain adhering to the inside of the mold, at which
      point the remaining liquid slip is poured out. For the finished molds to
      work, the gypsum needs to retain its porosity; hence, if that is the
      intent, I wouldn't have wax or any other oily material anywhere near the
      gypsum.

      Direct milling of gypsum blocks could work; it would bear some
      experimentation to see whether the milled surface was sufficiently "clean"
      (free from tool marks, pitting due to air bubble inclusions, surface
      fragmentation, etc) for the application in mind.
      The advantage of milling a "positive" and then casting the gypsum around it
      is that the resultant gypsum surface will (should) exactly replicate the
      surface of the positive, so once you have the positive finished to your
      satisfaction it should make a perfect mold every time.

      Regards,
      Tony



      On 3 July 2012 22:39, rpetrick2002 <rpetrick2002@...> wrote:

      > What about making a 'negative' in brass (or mild steel) to mold wax? And
      > then make lost wax molds in gypsum? The investment process is pretty dang
      > old and very reliable.


      [Non-text portions of this message have been removed]
    • Will Schmit
      I agree... I mill a hell-u-va-lot of molds, both male, and female (positive and negative), and I think that for ceramic molds, making a positive then pouring
      Message 2 of 8 , Jul 5 5:12 PM
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        I agree...
        I mill a hell-u-va-lot of molds, both male, and female (positive and negative), and I think that for ceramic molds, making a positive then pouring the plaster is the best way.  If nothing else, it allows you to set index pins in the half mold... old school, but proven for 2000 years.
        I have been spanked (pretty resoundingly) by porcelain.  I wish him luck.  I am not kidding (even a little bit), you will need to make 3 or 4 attempts, to sneek-up on the desired finished size and thickness.  It will be easier to make the changes in the design program, then re-mill a positive, then recast the plaster.



        ________________________________
        From: Tony Jeffree <tony@...>
        To: taigtools@yahoogroups.com
        Sent: Wednesday, July 4, 2012 7:17 AM
        Subject: Re: [taigtools] Re: milling gypsum ?


         
        If I interpret the OP's question correctly, he intends to make (female)
        gypsum molds that he will then use for slip-casting the porcelain;
        slip-casting involves pouring a slurry (slip) of the porcelain clay into
        the mold, leaving it there while the gypsum draws moisture out of the slip
        & leaves a layer of porcelain adhering to the inside of the mold, at which
        point the remaining liquid slip is poured out. For the finished molds to
        work, the gypsum needs to retain its porosity; hence, if that is the
        intent, I wouldn't have wax or any other oily material anywhere near the
        gypsum.

        Direct milling of gypsum blocks could work; it would bear some
        experimentation to see whether the milled surface was sufficiently "clean"
        (free from tool marks, pitting due to air bubble inclusions, surface
        fragmentation, etc) for the application in mind.
        The advantage of milling a "positive" and then casting the gypsum around it
        is that the resultant gypsum surface will (should) exactly replicate the
        surface of the positive, so once you have the positive finished to your
        satisfaction it should make a perfect mold every time.

        Regards,
        Tony

        On 3 July 2012 22:39, rpetrick2002 <rpetrick2002@...> wrote:

        > What about making a 'negative' in brass (or mild steel) to mold wax? And
        > then make lost wax molds in gypsum? The investment process is pretty dang
        > old and very reliable.

        [Non-text portions of this message have been removed]




        [Non-text portions of this message have been removed]
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